The present invention relates generally to methods and apparatus for selecting between or among cell streams in a communication network. More particularly, the present invention relates generally to methods for selecting between two or more redundant or competing cell streams from a cell-oriented environment, such as for example in an ATM (Asynchronous Transfer Mode) cell-based redundant switching system, connected to an external communications network.
Switching equipment may be designed with various layers of redundancy, including redundancy in the switch network portion of the switch itself. A switch network which provides redundancy is comprised of two or more redundant switch network copies. Such a redundancy scheme provides a vehicle whereby the “better” (also sometimes characterized in the art as the “best”) cell of two or more copies of a cell may be selected for insertion into the data stream. Providing at least two redundant or competing cell streams from which to choose the better cell, as determined based upon at least one measure of cell quality, ensures the integrity of the data and minimizes the number of cells that may be dropped as “bad.” Because different paths can be used to route different copies of a cell to the same destination, typically a decision must be made as to which of the copies received at that destination shall be further communicated in the network.
One prior art solution for selecting between redundant cell streams involves the use of control cells to synchronize cell streams that are redundantly transmitted in a cell-based communication network. These control cells, which contain a continuously changing sequence number, are inserted at regular intervals into the redundant cell streams at the start of the path. The presence of these control cells is monitored at the end of the path for each of the cell streams. The selected stream is identified, but it is not passed on until the sequence number in the selected stream matches the sequence number in its corresponding redundant stream. However, the cells are only aligned after a control cell is received; no alignment takes place in the time interval between arrival of control cells. Thus, dropped cells can cause misalignment until the next control cell is received. Further, a cell is not selected to be sent on to the customer network based on cell quality, i.e., the “best” of the received cells is not necessarily the cell sent on to the customer. Instead, the basis for selection is the presence or absence of a matching sequence number in the control cells of the two streams.
One objective of the present invention is to intelligently select one of two or more copies of a cell. According to one aspect of the invention, this selection is made by aligning the cell streams prior to selecting the best cell.
Another objective of the present invention is to tolerate a limited delay between the arrival of both cell copies, as well as a varying degree of bit errors associated with each of the cell copies.
Another objective of the present invention is to provide glitchless copy switching in a cell-oriented environment.
Another objective of the present invention is to select the best cell copy despite bit errors or other problems which may occur on one or both copies of the cell stream as the cell stream propagates.
Another objective of the present invention is to select the best cell copy while reducing or minimizing the number of cells dropped and maintaining the ordered nature of the cell stream.
These and other objects of the invention are discussed in or will be apparent to those skilled in the art from the following detailed description of the invention.